The invention relates to a novel combination of surface active agents for adjusting and stabilizing air voids in hydratable cementitious compositions.
As used herein, the term xe2x80x9cconcretexe2x80x9d means and refers to a mixture of a hydratable cementitious binder (e.g., Portland cement), fine aggregate (sand), and coarse aggregate (crushed stone or gravel) to which water is added to provide a composition that can harden into a building structure (e.g., foundation) or a civil engineering structure (e.g., tunnel) or other structural component.
In World Patent Application No. PCT/US98/12876, which is incorporated by reference herein, Jardine et al. disclosed that oxyalkylene-containing polymer additives, used as plasticizing or water-reducing admixtures in concrete, sometimes had unpredictable dosage behavior in different concrete mixes. Jardine et al. found that the aggregates contained in some concrete mixes had certain swellable clays (e.g., smectites) that negatively interacted with the plasticizer to create poor dosage response. It was theorized that such clays expanded when initially wetted by the mix water, and, in the expanded state, absorbed or entrapped the plasticizer. The invention of Jardine et al. was to employ clay-modifying agents. The purpose was to modify the clay activity, or mechanism, by which the clay-bearing aggregate interfered with the plasticizer activity, thereby to permit normal dosage of plasticizer to be used in the concrete.
The present inventors have discovered that, where an oxyalkylene-containing polyol polymer is used as the clay-modifying agent, the resultant concrete mix may have air entrainment problems, even when conventional air entraining agents (xe2x80x9cAEAsxe2x80x9d) are used. One of the objectives of the present invention, therefore, is to improve the air entrainment capability and air void quality of concrete mixes that employ oxyalkylene plasticizers and clay activity modifying agents. Another objective, more generally, is to provide a surface active agent system that is believed to be useful for adjusting and stabilizing air void systems in cementitious compositions, without seriously impeding the ability to employ conventional AEAs and/or clay-activity-modifying agents.
Technically speaking, an AEA does not generate air in the concrete, but merely stabilizes air that is (1) infolded (or mechanically enveloped) during mixing; (2) dissolved in the mix water; and/or (3) present in the intergranular spaces in the dry cement and aggregate. The concept of xe2x80x9centrainedxe2x80x9d air voids is different from the concept of xe2x80x9centrappedxe2x80x9d air voids. It is desirable (e.g., for purposes of increasing durability of hardened concrete under freeze-thaw conditions) to entrain air voids of a certain size (average diameter typically between 0.01-1.0 millimeter) and shape (generally spherical) in the paste portion of the wet concrete. Examples of common or conventional AEAs include water soluble salts (usually sodium) of wood resins, wood rosins, lignosulfonic acid, sulfonated hydrocarbons, proteinaceous materials, and fatty acids (e.g., tall oil fatty acid) and their esters.
The problem of controlling air void content and size distribution in concrete using AEAs is especially challenging when clay modifying agents, having a tendency to detrain intentionally entrained air, are present. When AEAs are added to concrete, they form a film at the interface between the air void and water phases, due to their varying solubility in the aqueous phase. (see e.g., xe2x80x9cAir Entraining Admixtures,xe2x80x9d from Concrete Admixtures, Dr. Vance Dodson (Van Nostrand Reinhold, N.Y. 1990), page 133-134). Furthermore, AEAs may be anionic, cationic, or nonionic in nature. Since common AEAs vary widely in composition and nature, a variation in their effectiveness might be expected. However, achieving consistency in managing the air content has been, for the most part, highly unpredictable.
The present invention pertains to a novel combination of surface active agents for adjusting and stabilizing the air void system in hydratable cementitious compositions such as concrete.
An exemplary surface active agent combination of the invention comprises (1) a first surface active agent, which is preferably employed in the amount of 0.0001-0.005% solids content by dry weight of the cement (xe2x80x9c% s/sxe2x80x9d) to be treated and which is preferably air entraining in the cementitious composition, this first surface active agent comprising a betaine, an alkyl and/or alkylaryl sulfonate, or mixture thereof; and (2) a second surface active agent, preferably in the amount of 0.0001-0.005% (by weight of the cement to be treated), the second agent comprising a nonionic oxyalkylene-containing polymer surfactant. Preferably, the nonionic oxyalkylene-containing polymer is a block copolymer.
The present inventors have discovered that the first surface active agent dramatically increases the air content but does not necessarily provide a fine and uniform air void distribution in concrete and does not, due to its sensitivity in small dosages, allow for easily-controllable air void volumes. Its combination with the second surface active agent, which operates to detrain air and thus improves fineness as well as the uniformity of air void distribution, attains a novel and fine-tunable air void system in a concrete. Thus, an exemplary admixture composition of the invention for a hydratable cementitious composition comprises a plasticizer having polyoxyalkylene groups; at least one clay activity modifying agent; a first surface active agent operative to entrain air in a hydratable cementitious composition; and a second surface active agent operative to render the air voids entrained by the first surface active agent more uniform in size.
One of the advantages of this combination is that the surface active agents can be used in relatively small dosages that permit conventional air entraining agents (AEAs) to be incorporated into the concrete (or other cementitious compositions) at conventional dosages and using conventional practices. This is also what is meant by the xe2x80x9cfine-tunabilityxe2x80x9d in concrete as provided by the admixtures of the invention.
The above-described combination of surface active agents is particularly useful for adjusting and stabilizing air void systems in concretes manufactured with a plasticizer (e.g., such as one comprising oxyalkylene groups and/or polycarboxylic acid groups or their salts or esters), one or more clay-modifying agents (for decreasing the plasticizer-absorbing activity of clay contained in the aggregate), and a conventional AEA. Thus, a further exemplary admixture composition of the invention comprises, in addition to the above-described surface active agent combination, a plasticizer (e.g., one comprising oxyalkylene groups and/or polycarboxylic acids or their salts or esters), and a clay-activity modifying agent. Optionally, though less preferably, the exemplary admixture composition may further comprise a conventional AEA.
An exemplary concrete composition or structure of the invention comprises a cementitious binder and the two surface active agents described above. Accordingly, a concrete of the invention comprises a Portland cement; at least one aggregate comprising fine aggregate, coarse aggregate, or mixture thereof; and the admixture of claim 1; said concrete further comprising, after said cement, at least one aggregate, and said admixture have been combined with water and allowed to harden, a uniformly distributed air void system having (A) a spacing factor less than 0.008 inch as measured in accordance with ASTM C457-98, and (B) a specific surface greater than 600 (1/inch) as measured in accordance with ASTM C457-98.
The invention also provides cementitious compositions containing said exemplary admixture composition, as well as methods for controlling air in concrete. The methods involve combining said admixture composition into a cementitious mixture.
Moreover, it is further believed that the use of the aforementioned two surface active agents leads to improved or enhanced xe2x80x9cfinishabilityxe2x80x9d (e.g.,the ability of the cementitious composition to be placed, smoothed, with excess water removed using minimal effort) in the concrete composition. Accordingly, a method of the invention for improving finishability of a wet concrete or masonry cement is to incorporate, into a wet hydratable cementitious mixture, the two surface agents described above.
Other advantages and features of the invention are discussed in further detail hereinafter.